1. Technical Field
The present invention relates to a motorcycle and a vehicle body frame for use in a motorcycle.
2. Description of the Background Art
A vehicle body frame for use in a motorcycle has a loop shape and stores an engine inside. The vehicle body frame is generally produced as an integral form. When a motorcycle is produced, an engine is stored in the vehicle body frame. Therefore, the vehicle body frame is larger than the engine.
If a vehicle body frame is too large, operational feeling about the motorcycle is degraded. If the vehicle body frame is small, the weight of the motorcycle is reduced, and the vehicle body frame as a whole is positioned in the vicinity of the center of gravity of the motorcycle. Therefore, good operational feeling is obtained. Therefore, smaller vehicle body frames are preferable.
Furthermore, a vehicle body frame must have balanced rigidity (stiffness). If the motorcycle is an off-road type vehicle such as a motocrosser in particular, the motorcycle travels off-road or jumps. Therefore, external force in various directions is applied on the vehicle body frame. If the rigidity greatly varies against external force in various directions, operational feeling is degraded. If for example the torsional rigidity is extremely small as compared to the tensile rigidity and compression stiffness, the vehicle body frame is likely to flex against force in a torsional direction. Therefore, if force is applied in a torsional direction, the vehicle body frame flexes more than usual. This makes the motorcycle less maneuverable. In this way, if the rigidity against external force in various directions is greatly unbalanced, operational feeling is degraded. Therefore, it is preferable to reduce unbalance in rigidity against different kinds of external force.
The disclosure of JP-A 2008-222077 and the disclosure of JP-A 4-244492 suggest a vehicle body frame that allows rigidity and strength to be secured. The vehicle body frames disclosed by JP-A 2008-222077 and JP-A 4-244492 are partly detachable. By selecting a material for the detachable frame part, sufficient strength and rigidity can be secured for the vehicle body frame.